In certain types of machines, such as light gas turbines for aircraft, dampers in the form of squeeze film dampers have long been a standard solution. This type of damper has also proved to be useful in larger machines as well.
If two surfaces with a viscous medium therebetween, for example oil, are brought together at a certain speed, a pressure build-up in the oil is obtained which is proportional to the speed. A squeeze film damper utilizes an oil film as damping agent between two concentric surfaces. In squeeze film dampers the pressure build-up is obtained between the concentric surfaces when the surfaces are vibrating relative to each other. Thus, the damper resembles a cylindrical bearing, with the difference that the internal part of the damper is not rotating. The internal part of the damper consists of the shaft bearing of the machine, which may be of any arbitrary type. In case of vibrations in the shaft, the bearing, i.e. the internal part of the damper, moves in a more or less circular path, which results in the oil film experiencing a squeezing travelling along the circular oil film between the concentric surfaces formed by the external surface of the internal part of the damper and the surrounding internal surface of the holder of the damper. The squeezing generates a rotating pressure front inside the circular oil film. This pressure front acts in a damping manner on the vibrations of the shaft. The machine reacts with lower vibration amplitudes, increased stability and, hopefully, with reduced bearing stresses.
Nowadays, squeeze film dampers are standard machine elements on, for example, gas turbine shafts journalled in roller bearings. They exist in two versions with centered or non-centered mounting. The centered damper is generally coupled with a spring means. The spring means retains the internal part of the damper centered and thus supports the weight of the shaft and the load thereof. The non-centered damper is positioned loosely and is only prevented from rotating. In both types of dampers, the oil, which builds up a squeeze film with a rotating pressure front, is supplied through radial holes bored in a holder for the damper to an annular groove, running along the periphery of the entire damper, in the form of a recess in either the internal part of the damper or in its holder. Because of the pressure of the oil, oil is pressed out from the mentioned groove to the annular gap between the internal part of the damper and its holder. The clearance in this gap is of the order of size of 0.25 mm. Since oil is pressed out of the groove on both sides in the axial direction, a thin oil film arises on both sides of the groove in the damper. This oil film constitutes the viscous medium of the damper itself for the rotating pressure front, the task of the medium being to counteract loads arising through vibrations.
The oil pressure to the damper may be of great importance for the dynamic stiffness of the damper.
In damper designs with a centered damper, the supporting spring is a critical component, especially for large machines. A desirable development of centered dampers would be to exclude the spring means, while at the same time maintaining its centering function.